A fan control unit that controls a cooling fan of an on-board heat exchanger is provided under the control of a vehicle ECU (Electronic Control Unit), and the cooling fan is controlled by controlling the rotation speed of a motor thereof according to the duty ratio of a PWM (pulse width modulation) signal that is output from the vehicle ECU.
The rotation speed of the motor is controlled based on the vehicle speed, the temperature of the coolant for the engine, and the AC pressure. In the case where the air conditioner is ON, the air-conditioning ECU calculates the necessary fan control and outputs the result to the vehicle ECU based on a pressure signal of the air conditioner and a vehicle speed signal received from the vehicle side. The vehicle ECU determines the rotation speed for the fan motor in consideration of the vehicle speed and the temperature of the coolant for the engine in addition to the signal and outputs a fan-driving PWM signal. In the case where the air conditioner is OFF, the vehicle ECU determines the rotation speed of the fan based on the vehicle speed and the temperature of the coolant for the engine and outputs a fan-driving PWM signal.